1. Field of the Invention
The present invention relates to an air-fuel ratio feedback control method for an internal combustion engine which can be operated with the use of any of at least two fuels, such as a gasoline fuel, alcohol fuel, etc.
2. Description of the Related Art
In consideration of the anticipated exhaustion of petrochemical fuel resources and in order to improve the exhaust gas properties, intense research has recently been made in internal combustion engines which can use a simple gasoline or alcohol fuel or a mixture of these two fuels as their working fuel.
The conventional multi-fuel internal combustion engines are designed so that the amount of fuel supply (injection quantity) is only adjusted in accordance with the mixture ratio of the fuel, and they are basically the same as the conventional gasoline engines with respect to the air-fuel ratio feedback control method and exhaust gas purifier.
For the internal combustion engines using the gasoline fuel, a so-called dual O.sub.2 sensor system has been proposed to cope with the regulations on exhaust gas which have been becoming stricter every year. This system includes O.sub.2 sensors which are arranged individually on the up- and downstream sides of a catalytic converter in an exhaust passage and used to detect the concentration of oxygen in exhaust gas. Air-fuel ratio feedback control by means of this system is effected by setting a feedback correction factor value KFB, on which the fuel injection quantity depends, in the following manner, for example. If an output voltage VO.sub.2 of a front O.sub.2 sensor on the upstream side shifts across a reference voltage Vs to the lean side, a proportional term value P is added to the value KFB, and an integral term value .DELTA.I is added thereafter with every passage of a predetermined time or with every rotation of a crankshaft for a predetermined crank angle. If the output voltage VO.sub.2 of the front O.sub.2 sensor shifts across the reference voltage Vs to the rich side, on the other hand, the proportional term value P is subtracted from the air-fuel ratio correction factor value KFB, and the integral term value .DELTA.I is subtracted thereafter with every passage of the predetermined time or with every detection of a predetermined crank angle position signal. In response to an output from a rear O.sub.2 sensor on the downstream side, moreover, the value of the reference voltage Vs is adjusted.
This system should be also applied to the multi-fuel internal combustion engines. If the fuel mixture ratio changes, however, the exhaust gas ingredients vary despite the fixed excess air ratio (or air-fuel ratio), so that the value of the output of the rear O.sub.2 sensor inevitably changes. Thus, the dual O.sub.2 sensor system cannot properly function, so that the air-fuel ratio feedback control cannot be achieved with high accuracy.